This invention relates to an improved lateral pressure detector for use with a phonograph record player.
FIG. 1 illustrates the general phonograph playing state of a tone arm and pick up cartridge. The line 2-4 linking the fulcrum 2 of the tone arm with the needle tip 4 of the cartridge and the center line 3-L of the cartridge 3 are offset from one another by an angle .theta.. This is so selected as to minimize the angle of error between center line 3-L of the cartridge and the sweeping direction of the sound groove 5 at needle tip 4 in the effective length of the tone arm 1. By means of this, the error (horizontal tracking error) is held to .+-.3.degree. from the outer circumference to the inner circumference of a 30 cm LP record. In the type of offset arm which is used most at present, when the needle pressure applied to needle tip 4 is selected as Fs at the condition of negligible horizontal tracking error, a friction force F=.mu.Fs develops between needle tip 4 and sound groove 5 (.mu. is the sliding friction coefficient between the needle tip and the sound groove) in the sweeping direction of sound groove 5. Since the direction of this friction force F when seen from the fulcrum 2 of the tone arm deviates only by the offset angle .theta., it operates as a pulling force F cos .theta. on tone arm 1 containing cartridge 3 in the direction of line 2-4 or as a force F sin .theta. which revolves tone arm 1 toward the inner side of record 6. Since force F sin .theta. operates as a lateral pressure of cartridge 3, it is generally termed an inside force. In order to negate this force, a force which rotates the tone arm toward the outer circumference of the record, specifically the imposition of outside force 7, is necessary. This is generally accomplished by an outside force imposition device known as an inside force canceller or an anti-skating device.
The inside force F sin .theta. can be expressed as .mu.Fs .multidot. sin .theta. as a result of the above formula for needle pressure. Specifically, since the inside force changes relative to changes in the needle pressure, the amount of outside force imposed in the anti-skating device also must change in response to the needle pressure. Moreover, since the stress received by the record at the contact area of the record groove changes with the shape of the needle tip used, the value of the slide friction coefficient .mu. differs with round needles, oval needles or the newer line contact needles. Accordingly, the amount of outside force imposed must also vary with the shape of the needle.
In order to determine the appropriate amount of outside force to impose, it is known to provide a record for measuring the cartridge trace having a signal with a fast amplitude velocity near the limit which the cartridge used can trace. An outside force is provided so that the output signals on the left and right channels reach the trace limit at the same time and the force is thus determined. Also see U.S. Pat. No. 3,328,037 for an apparatus for measuring the anti-skating force in pick ups.
Since the above method uses signals near the trace limit of the cartridge, values are obtained which differ from actual values of the mechanical impedance of the left and right channels of the oscillation system. In addition, since the cartridge is at the trace limit, it readily accepts resistance in the form of revolving frictional force at the fulcrum of the tone arm. Another problem concerns the material of the test record which has values different from those of ordinary records.